Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Chiral Phthalocyanines through Axial Coordination Dora M. Răsădean, Tiberiu M. Gianga, Alexander H. Swan, Gabriele Kociok-Köhn, and G. Dan Pantoş* Department of Chemistry, University of Bath, Bath, BA2 7AY, U.K. S Supporting Information *
ABSTRACT: A novel approach to axially induce chirality on silicon phthalocyanines via a microwave-assisted route is reported. CD analysis provides spectroscopic evidence that chirality is transferred onto both Soret and Q-bands of the phthalocyanine core. A chiral naphthalenediimide ligand was found to induce the largest Cotton effect on the macrocycle absorptions.
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Scheme 1. General Reaction Protocol for Synthesis of Chiral Phthalocyanines 1−9 (Color Codes: Red, Disubstituted Pcs; Blue, Monosubstituted Pcs)
hirality plays a central role in science, spanning from liferegulating chemical processes and drugs to catalysis and supramolecular optical devices.1,2 The insertion of chirality into planar achiral entities has been proposed as a powerful and accessible approach for tailoring optically active devices.3−7 In light of this, phthalocyanines (Pcs), albeit intrinsically achiral, have emerged as ideal and versatile synthetic building blocks due to their unique electronic and spectral features.8−10 Because of their thermal stability, chemical robustness, and ability to self-assemble,11 a plethora of synthetic routes toward generating optically active phthalocyanine derivatives have been put forward.12,13 The current methods incorporate stereocenters into the peripheral positions of the Pc core14,15 or involve the introduction of substituents that distort the symmetry of the Pc macrocycle.16 These synthetic routes are laborious, mostly low yielding and employ harsh reaction conditions.10,17−19 An alternative approach to producing optically active Pcs consists of axial functionalization,20−24 thus taking advantage of the free coordination sites of the metal cation. This has been less explored; to date only a few Sn and Ti13 derivatives and an atropisomeric Zn-Pc complex with binaphthyl units25 have been reported. While a few Si-Pcs26−29 bearing chiral carboxylate units30,31 on the Si coordination sites are reported in the literature, no study, or even observation, of their chiral nature is provided. The published syntheses of these Si-Pcs30,31 require harsh conditions (e.g., strong bases such as NaH, DBU) and prolonged reaction times that limit the number of chiral groups that can be appended using this method. In order for chiral Pcs to be considered for optically active devices, a more direct and high yielding synthesis is necessary. We report herein a rationally designed approach for the efficient synthesis of chiral Si-Pcs (Scheme 1), as well as a comprehensive analysis of their absorption, fluorescence, and circular dichroism properties. We have developed a one-pot, microwave-assisted protocol that uses the non-nucleophilic Hünig’s base and short reaction times and produces moderate © XXXX American Chemical Society
to good yields of axially coordinated, chiral Si-Pcs. The synthetic route, depicted in Scheme 1, involves an exchange between chloride and carboxylate ligands on the Si center. Mono- and dicarboxylate derivatives have been synthesized by varying the amount of carboxylic acid added to the Received: March 15, 2018
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DOI: 10.1021/acs.orglett.8b00851 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters commercially available PcSiCl2. Monocarboxylate-monochloro Si-Pcs are attractive, not only because of their chirality but also because of the desymmetrization of the Pc core, which is then amenable for further functionalization. Table 1 summarizes the reaction conditions and yields obtained for each derivative. Table 1. Reaction Conditions and Yields for Pcs1−9 product name
equiv of ligand
Pc1 Pc2 Pc3 Pc4 Pc5 Pc6 Pc7 Pc8 Pc9
5 1 5 1 5 5 5 5 5
reaction conditionsa
yield (%)
°C, °C, °C, °C, °C, °C, °C, °C, °C,
68 15 83 14 63 6 33b 42 51
155 125 155 125 155 155 155 155 155
14 h 3h 14 h 3h 14 h 14 h 14 h 14 h 14 h
a
All reactions were performed in dry toluene/DIPEA in the microwave as shown in Scheme 1. Detailed protocols for each derivative are in the Supporting Information (SI). bThis yield corresponds to a mixture of two conformationally different disubstituted derivatives.
The axial ligands have been selected based on the position of the stereocenter with respect to the ligation site (proximal or distal from the Pc core), as well as based on their level of saturation (aromatic and aliphatic scaffolds) in an effort to establish the best structural elements for inducing chirality on the Pc chromophore. The typical 1H NMR signature of an axially substituted Pc displays a large upfield shift of the signals, corresponding to the protons closest to the Pc ring (i.e., the protons in the α position relative to the carboxylate moiety). For example, in the case of Pc8, the dd (CH) at 2.86 ppm corresponding to the proton next to the carboxylate group (Figure 1, top) is the closest to the Pc unit, hence the most shielded with respect to its unligated chemical shift. This molecule (Pc8) exhibits unique spectral features, as half of the naphthalenediimide (NDI) moiety is in the proximity of the Pc core, thus explaining the upfield shift (δ 7.49 ppm) of these protons, compared to the rest of NDI’s protons, which appear slightly deshielded (δ 8.39 ppm). The 1H NMR spectra also provide a valuable tool in determining the ligand/Pc ratio; the 2:1 or 1:1 stoichiometry has been easily established based on the splitting pattern of the aromatic protons corresponding to the Pc ring, as well as the integration values of the ligand/Pc signals. Single crystals, suitable for X-ray diffraction analysis, of Pc8 have been obtained by slow evaporation of an acetonitrile solution. The molecule crystallizes in the tetragonal space group P43212 with the carboxylate scaffolds arranged in trans geometry, and an NDI moiety parallel with the Pc core, as illustrated in Figure 1a. Each NDI unit is packed between two Pcs, assembling into an extended aromatic π−π stacking fashion (Figure 1b). There are weak C−H···OC interactions (2.98(2) Å, 137°) between the adjacent NDI units within the structure. The structure of Pc5 was also confirmed by X-ray diffraction crystallography of single crystals grown from CHCl3. The molecule crystallizes in the orthorhombic space group P212121 with the carboxylate moieties arranged in trans geometry with respect to each other (Figure 2). There are aromatic stacking interactions between the axial ligand’s aromatic moiety and Pc
Figure 1. Top: the structure and labeled 1H NMR spectrum of Pc8 in CDCl3 at room temperature. Bottom: (a) ORTEP representation of Pc8. The thermal ellipsoids were drawn at 50% probability level (Hs were removed for clarity); (b) fragment of the extended aromatic π−π stacking in the crystal packing of Pc8.
Figure 2. (a) ORTEP representation of Pc5. The thermal ellipsoids were drawn at 50% probability level (Hs were removed for clarity); (b) fragment of the herringbone extended aromatic π−π stacking in the crystal packing of Pc5.
macrocycle with an interplanar distance of 3.351 Å, and between the Pc cores of two neighboring molecules, with a distance of 3.379 Å. Two separate stacks are arranged in a herringbone-like fashion, as illustrated in Figure 2. Circular dichroism (CD) is generally recognized as a valuable tool in assessing the chiral nature of a molecule and, together with UV−vis spectroscopy, provides unique insight into the optoelectronic and structural properties of chiral molecules.32 The absorption spectra recorded in CH2Cl2 indicate that all of our derivatives exhibit the typical spectral features of phthalocyaninato-metal complexes (typical CD and UV−vis spectra are illustrated in Figure 3 for Pc9), showing their nonaggregated nature (by sharpness of the band). The intense absorption band around 685 nm is the Pc’s signature Q-band B
DOI: 10.1021/acs.orglett.8b00851 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Figure 4. (a) Overlaid UV−vis spectra (CH2Cl2, 20 °C) of Pc1 and Pc3; (b) normalized emission of Pc1 and Pc3 in CH2Cl2 excited at the wavelengths described in the legend. Figure 3. CD (red) and UV−vis (blue) spectra of Pc9 in CH2Cl2 (2.92 × 10−5 M).
695 and 684 nm, respectively), which is potentially attributed to the intramolecular aromatic stacking interactions between the NDI and Pc cores restricting the molecular motions, as observed in the crystal structure of Pc8 (Figure 1, bottom). The other aryl-based disubstituted derivatives (Pcs1 and 5) have also exhibited an excellent ability to induce chirality due to their propensity to form aromatic stacking with the Pc ring; the crystal structure of Pc1 previously reported by Sosa-Sánchez et al. confirms this hypothesis.30 The position of a stereocenter relative to the coordination site is highly influential, as can be seen in the lower molar ellipticity exhibited by Pc7, where the stereocenter is far away from the Pc ring. The only aliphatic ligand (isoleucine) of the disubstituted molecules series has one of the weakest CD signals, albeit its stereocenter is also located in the β position with respect to the Pc. This consolidates our assumption that the presence of the aromatic moieties is beneficial for generating chirality on the Pc’s Q-band. The fully desymmetrized monosubstituted analogues (Pcs2, 4, 6, 9) are generally less soluble and more prone to aggregation than the disubstituted derivatives. The Q-band extinction coefficients of all the mono derivatives are an order of magnitude lower than their disubstituted counterparts. A similar trend is observed for the molar ellipticities measured on the Q-band absorbance (Table 2 and SI). The emission and excitation spectra of Pcs in CH2Cl2 display similar spectral characteristics, except for Pcs 8 and 9 which can be considered anomalies due to the fluorescence quenching nature of the NDI core. All fluorescence spectra are provided in the SI, and Table 2 summarizes the emission maxima upon excitation of the Q-band, along with the quantum yields calculated using Rhodamine 800 as the standard.
and corresponds to the lowest allowed π → π* transition;13 a weak vibrational shoulder at around 618 nm is also present.13,17 The disubstituted derivatives are, as expected, slightly redshifted, compared to the monocarboxylate ones, with Pc8 having the most red-shifted absorbance (λmax = 695 nm). The Soret band region displays weak broad bands across 332−382 nm that arise from π → π* and n → π* transitions.13,33 The NDI-functionalized Pc, Pc8, exhibits the most intense absorption in the Soret-band region, which accounts for the overlapping signals of Pc and the NDI core. All the UV−vis spectra of the Pc derivatives studied are provided in the SI, and spectral features are summarized in Table 2. The spectroscopic fingerprints of our Pcs are consistent with previously reported data (Figure 4).13,17,33,34 Table 2 provides a direct assessment of the chiroptical properties of each mono-/disubstituted Pc pair. The CD spectra (provided in the SI) display a Cotton effect around 680 nm, which corresponds to the Q-band of the absorption spectra. This confirms that the chiral information is effectively transferred from the axial ligand to the Pc core. Without the axial chiral ligand, the Pc chromophore is in-plane polarized, and hence intrinsically achiral, and would not have shown any CD signal at these wavelengths.35 As expected, the Soret-type absorbance also shows a Cotton effect under the influence of the axial ligands. The comparison of the molar ellipticities around 680 nm exhibited by the chiral Si-Pcs allowed us to identify the best ligand for chirality induction on the Pc chromophore. The NDI-functionalized Pcs (Pcs8 and 9) have the largest molar ellipticities (4.436 × 104 and −4.907 × 104 deg·cm2·dmol−1 at Table 2. Spectral Characteristics of Pcs1−9
emissionb
Q-band characteristics product
λmax (nm)
εa (L·mol−1·cm−1)
molar ellipticity ×104 (deg·cm2·dmol−1)
λmax (nm)
Φ
Pc1 Pc2 Pc3 Pc4 Pc5 Pc6 Pc7 Pc8 Pc9
688 682 688 676 685 678 686 695 684
234 000 60 000 259 000 24 600 74 000 3400 62 000 115 000 50 000
−2.742 −0.293 −0.749 0.807 1.960 1.019 0.522 4.436 −4.907
693 683 693 677 690 679 693 673 671
0.77 n/ac 0.79 n/ac n/ac n/ac 0.72 0 0.28
For error margins, see Table S1 in the SI. bλmax while exciting the Q-band. cNot available due to aggregation issues. The Φ of Pc8 cannot be calculated because of the two NDI-based axial ligands, which are fluorescence quenching.
a
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DOI: 10.1021/acs.orglett.8b00851 Org. Lett. XXXX, XXX, XXX−XXX
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(7) Zysman-Colman, E.; Ghosh, S. S.; Xie, G.; Varghese, S.; Chowdhury, M.; Sharma, N.; Cordes, D. B.; Slawin, A. M. Z.; Samuel, I. D. W. ACS Appl. Mater. Interfaces 2016, 8, 9247. (8) Bottari, G.; de la Torre, G.; Torres, T. Acc. Chem. Res. 2015, 48, 900. (9) Lv, W.; Zhang, X.; Lu, J.; Zhang, Y.; Li, X.; Jiang, J. Eur. J. Inorg. Chem. 2008, 2008, 4255. (10) Kobayashi, N. Synthesis and Characterization of Chiral Phthalocyanines. Handbook of Porphyrin Science (Vol. 23); World Scientific Publishing Company: Singapore, 2012; Vol. 25, p 373. (11) Zhang, W.; Fujiki, M.; Zhu, X. Chem. - Eur. J. 2011, 17, 10628. (12) de la Torre, G.; Torres, T.; Agulló-López, F. Adv. Mater. 1997, 9, 265. (13) Kobayashi, N. Coord. Chem. Rev. 2001, 219−221, 99. (14) Mack, J.; Kobayashi, N. Chem. Rev. 2011, 111, 281. (15) Kobayashi, N.; Kobayashi, Y.; Osa, T. J. Am. Chem. Soc. 1993, 115, 10994. (16) Kobayashi, N.; Narita, F.; Ishii, K.; Muranaka, A. Chem. - Eur. J. 2009, 15, 10173. (17) Fox, J. M.; Katz, T. J.; Van Elshocht, S.; Verbiest, T.; Kauranen, M.; Persoons, A.; Thongpanchang, T.; Krauss, T.; Brus, L. J. Am. Chem. Soc. 1999, 121, 3453. (18) Lu, H.; Kobayashi, N. Chem. Rev. 2016, 116, 6184. (19) Bottari, G.; de la Torre, G.; Guldi, D. M.; Torres, T. Chem. Rev. 2010, 110, 6768. (20) Martin-Gomis, L.; Ohkubo, K.; Fernandez-Lazaro, F.; Fukuzumi, S.; Sastre-Santos, A. J. Phys. Chem. C 2008, 112, 17694. (21) Taratula, O.; Schumann, C.; Naleway, M. A.; Pang, A. J.; Chon, K. J.; Taratula, O. Mol. Pharmaceutics 2013, 10, 3946. (22) Rotas, G.; Martin-Gomis, L.; Ohkubo, K.; Fernandez-Lazaro, F.; Fukuzumi, S.; Tagmatarchis, N.; Sastre-Santos, A. Chem. - Eur. J. 2016, 22, 15137. (23) Fukuzumi, S.; Honda, T.; Ohkubo, K.; Kojima, T. Dalton Trans. 2009, 20, 3880. (24) Sun, Q.; Zheng, B.-Y.; Zhang, Y.-H.; Zhuang, J.-J.; Ke, M.-R.; Huang, J.-D. Dyes Pigm. 2017, 141, 521. (25) Kobayashi, N.; Muranaka, A.; Ishii, K. Inorg. Chem. 2000, 39, 2256. (26) Martin-Gomis, L.; Peralta-Ruiz, F.; Thomas, M. B.; FernandezLazaro, F.; D’Souza, F.; Sastre-Santos, A. Chem. - Eur. J. 2017, 23, 3863. (27) Martin-Gomis, L.; Ohkubo, K.; Fernandez-Lazaro, F.; Fukuzumi, S.; Sastre-Santos, A. Chem. Commun. 2010, 46, 3944. (28) Arellano, L. M.; Martín-Gomis, L.; Gobeze, H. B.; Barrejón, M.; Molina, D.; Gómez-Escalonilla, M. J.; Fierro, J. L. G.; Zhang, M.; Yudasaka, M.; Iijima, S.; et al. J. Mater. Chem. C 2015, 3, 10215. (29) Martin-Gomis, L.; Ohkubo, K.; Fernandez-Lazaro, F.; Fukuzumi, S.; Sastre-Santos, A. Org. Lett. 2007, 9, 3441. (30) Sosa-Sanchez, J. L.; Galindo, A.; Gnecco, D.; Bernes, S.; Fern, G. R.; Silver, J.; Sosa-Sanchez, A.; Enriquez, R. G. J. Porphyrins Phthalocyanines 2002, 6, 198. (31) Rodriguez-Cordoba, W.; Noria, R.; Guarin, C. A.; Peon, J. J. Am. Chem. Soc. 2011, 133, 4698. (32) Muranaka, A.; Yoshida, K.; Shoji, T.; Moriichi, N.; Masumoto, S.; Kanda, T.; Ohtake, Y.; Kobayashi, N. Org. Lett. 2006, 8, 2447. (33) Zhou, H.; Wang, K.; Qi, D.; Jiang, J. Dalton Trans 2014, 43, 1699. (34) Tian, J.; Jing, L.; Ji, L.; Zhang, C.; Liu, Q.; Zhang, X. RSC Adv. 2013, 3, 22461. (35) Kobayashi, N.; Higashi, R.; Titeca, B. C.; Lamote, F.; Ceulemans, A. J. Am. Chem. Soc. 1999, 121, 12018.
In summary, we report a novel approach to induce chirality via the axial positions onto intrinsically achiral Pcs. We have synthesized a series of mono- and disubstituted chiral Si-Pc derivatives, using a one-pot microwave-assisted protocol. The synthesis employs milder conditions and results in superior yields compared to the traditional methods. The optical and chiroptical properties of these molecules make them interesting candidates for chiral optoelectronic applications. The monocarboxylate-monochloro Si-Pc derivatives are particularly interesting because of their total asymmetry, which can be exploited for further functionalization.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00851. Experimental procedures and spectroscopic data for all the compounds discussed in the manuscript (PDF) Accession Codes
CCDC 1578829−1578830 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
G. Dan Pantoş: 0000-0003-2200-550X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by the GW4 consortium (GW4-AF5002) and the University of Bath. D.M.R. thanks Mr. Yuto Kage of Kyushu University for useful discussions about fluorescence. X-ray diffraction and NMR facilities were provided through the Chemical Characterisation and Analysis Facility (CCAF) at the University of Bath, while the mass spectrometry data were acquired at the EPSRC National Mass Spectrometry Facility at Swansea University.
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REFERENCES
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DOI: 10.1021/acs.orglett.8b00851 Org. Lett. XXXX, XXX, XXX−XXX
